California Confirms New Oak Disease

Quercus lobata – California’s valley oak trees – are struggling against a new form of disease that manifests in root rot, decline and stunting. The culprit? Phytophthora quercina (not to be confused with Phytophthora ramorum) has been determined to be the causal agent, and it’s been confirmed by USDA’s Animal and Plant Health Inspection Service as having a foothold in Santa Clara County – so far, the only such report of the disease in the U.S.

It appears that the host range is limited to oaks; P. quercina is well- established in Europe in nine species but, according to the Horticultural Research Institute’s Jill Calabro, the full extent of susceptibility among oak species is not known. Prior to its detection in California, P. quercina was not known to affect Quercus lobata.

What is known is that Phytophthora quercina is water- and soil-borne, which just makes it easy to spread through all manner of pathways: infested soil, tools and equipment that have come into contact with infected plants, other planting stock.

So be on the lookout: Symptoms resemble other Phytophthora diseases, and the distressing fact is that a tree affected by P. quercina can appear to be drought-stressed. Watch for branch dieback and visible canopy thinning; epicormic shoots and wilting leaves. If you can check belowground, you may see necrotic lesions on roots, weird root branching or deterioration of the finer roots.

Boxwood Blight Quarantine in Pennsylvania

The Pennsylvania Department of Agriculture (PDA) in June imposed a quarantine “on any property, facility, premises, place or area where Boxwood Blight is confirmed.” This internal and external quarantine is the result of several detections of the disease on nursery stock in 2015, and it prevents movement of Buxus both within and into the Commonwealth.

According to the PDA order, regulated articles include any living life stage of the Boxwood Blight (Calonectria pseudonaviculatum); all plants and plant parts; brush, debris, bark or yard waste; landscaping, nursery/greenhouse and plant retail waste; and leaves, branches and soil of infected Buxus genus.

The PDA is urging nurseries and other green industry professionals who handle boxwood to comply with the best management practices recommended by AmericanHort; these guidelines were developed in 2012 by then-ANLA and the National Plant Board.